Co-axial loudspeakers are designed with a high frequency drive unit positioned at or adjacent to the neck of the diaphragm of a low frequency drive unit, as shown in U.S. Pat. No. 5,548,657 and FIG. 1 of the accompanying drawings. As a result, the apparent sound source or acoustic centre of the high frequency drive unit is substantially co-incident with the apparent sound source or acoustic centre of the low frequency drive unit. With the high frequency drive unit positioned adjacent to the neck of the low frequency diaphragm, the form of the low frequency diaphragm imposes its directivity (if any) upon the radiation pattern or directivity of the high frequency unit. Consequently at frequencies at which both drive units contribute significant sound output, both drive units have substantially similar patterns of radiation or directivity. As a result the relative sound contributions from the two drive units, as perceived by a listener, are substantially unaffected by the listener being positioned at off axis positions. Such arrangements have become well known since U.S. Pat. No. 5,548,657 in the form of our UNI-Q™ speaker.
Referring to FIG. 1, the compound loudspeaker drive unit with low frequency and high frequency transducers having co-axial low and high frequency voice coils comprises a chassis 10 in the form of a conical basket having a front annular rim 11 connected to a rear annular member 12 by means of a number of ribs 13. The rear annular member 12 has an annular flange 14 and an annular seat 15. Secured to the flange 14 is a first magnetic structure 16 for the low frequency loudspeaker drive unit. The magnetic structure 16 comprises a magnet ring 17, a front annular plate 18 which forms an outer pole and a member which forms a backplate 19 and an inner pole 20. The plate 18, magnet ring 17 and member are held together to provide a magnetic path interrupted by a non-magnetic air gap between the outer pole 18 and the inner pole 20. The poles are circular and form therebetween an annular air gap. The low frequency transducer or loudspeaker drive unit comprises a diaphragm 21 of generally frusto-conical form supported along the front outer edge thereof by a flexible surround 22 secured to the front rim 11 of the chassis 10. A tubular coil former 23 is secured to the rear edge of the diaphragm 21 and is arranged to extend co-axially of the air gap in the magnetic structure 16. The coil former carries a voice coil 24 positioned on the former such that the coil extends through the air gap. The coil is of sufficient axial length as to ensure that for normal excursions of the voice coil, the poles always lie within the length of the voice coil. A suspension member 25 is secured between the coil former 23 and the annular seat 15 of the chassis 10 in order to ensure that the coil former, and voice coil carried thereby, are maintained concentric with the poles of the magnetic structure and out of physical contact with the poles during sound producing excursions of the diaphragm 21. The member forming the backplate 19 and inner pole has a bore 26 extending co-axially thereof for the purpose of mounting a high frequency drive unit 27.
The high frequency transducer or drive unit 27 comprises a second magnetic structure consisting of a pot 28, a disc shaped magnet 29 and a disc shaped inner pole 30. The pot 28 has a cylindrical outer surface so dimensioned as to fit within the interior of the coil former 23 without making physical contact therewith. The pot is formed with a circular recess 31 to receive the magnet 29 and an annular lip 32 to form an outer pole. One circular pole face of the magnet 29 is held in engagement with the bottom wall of the recess 31 and the disc shaped inner pole 30 is held in engagement with the other circular pole face of the magnet such that the circular outer periphery of the inner pole 30 lies co-axially with and within the lip 32 forming the outer pole. An air gap extends between the inner and outer poles. A spacer ring 33 is secured to the front face of the pot 28. A high frequency domed diaphragm 34 has an annular support 35 secured at its outer periphery to the spacer ring 33. Secured to the domed diaphragm 34 is a cylindrical coil former carrying a high frequency voice coil 36 such that the voice coil extends through the air gap between the poles 30, 32 of the magnetic structure.
As a result of the coaxial design, such loudspeakers have an annular gap 40 extending axially between the high frequency unit 27 and the midrange voice coil former 23. This gap is necessary to provide clearance so the midrange voice coil can move freely without touching the tweeter body. However, it defines a generally cylindrical channel 44 around the high-frequency unit 27 which allows some unwanted acoustic resonances to take place, causing irregularities in the high frequency response.
Existing coaxial drivers are mostly designed to minimize this volume of air and keep the width of the gap between the tweeter and the midrange cone as small as possible. Cylindrical inserts have been placed in the gap, to reduce its overall volume. A different approach that has been adopted is to separate the air channel with a flexible seal, such as in US 2013/0142379 which describes a small flexible surround covering the air gap between the tweeter and the midrange drivers. This approach prevents the resonances inside the air channel from affecting the high frequency response of the unit, but in order to present a smooth waveguide for the tweeter this additional surround must be conical or very small. As a result, its stiffness varies strongly with displacement thereby causing harmonic distortion and limiting the maximum sound pressure level of the midrange driver. Other designers have incorporated a large half roll rubber surround between the high frequency unit and midrange cone; this introduces a large physical discontinuity to the waveguide instead, and will introduce significant diffraction to the high frequency response of the unit.